Darío Orts

A Review of the Geology, Structural Controls, and Tectonic Setting of Copahue Volcano, Southern Volcanic Zone, Andes, Argentina

Copahue Volcano lies in the Southern Volcanic Zone of the Andes Mountains, although its geology and local structural controls differ from nearby active volcanic centers. Most of its geology is substantially older than active volcanoes at these latitudes, as the postglacial component is relatively minor. The basement of Copahue Volcano, represented by the Agrio Caldera products and its basal sections, accumulated in extensional depocenters when the arc narrowed from a broad geometry on both sides of the Andes to its present configuration. Initial stages comprise early Pliocene basaltic-andesitic eruptions associated with extensional (transtensional?) processes that ended with the formation of a series of rhombohedral calderas that emitted important amounts of ignimbrites in latest Pliocene-early Pleistocene time. Copahue Volcano concentrates the Pleistocene activity of one of these calderas, the Agrio Caldera, before the emplacement and development of the Present arc front to the west. Volcano morphology reflects this particular evolution, looking more degraded than Antuco, Callaqui and Lonquimay volcanoes located immediately to the west in the arc front. Most of Copahue’s volume is early Pleistocene in age, showing a thin resurfacing cover in synglacial (>27 ka) and postglacial times. A synglacial stage occurred mainly to the east of Copahue Volcano toward the caldera interior in a series of independent, mostly monogenetic centers. Postglacial eruptions occurred as both central and fissural emissions reactivating the old Pleistocene conduits. Its particular geological record and eastern longitudinal position indicate that Copahue was probably part of the late Pliocene-Pleistocene arc mostly developed in the axial and eastern Andes. Narrowing and westward retraction of the arc front, proposed in previous works for the last 5 Ma at 38°S, could have been the result of the eastward migration of the asthenospheric wedge during slab steepening. Reasons for this long-lived eruptive history at Copahue volcano could be related to the particular geometry of the active Liquine-Ofqui dextral strike-slip fault system that runs through the arc front from south to north when penetrates the retroarc area at the latitude of Copahue volcano. This behavior could be due to the collision of the oceanic Mocha plateau at these latitudes, as recently proposed. This jump and related deflection would have produced local transtensional deformation associated with abundant emissions of syn- and post-glacial products that could have partially resurfaced this long-lived center.

Growth of the Southern Andes

GOCE satellite data and EGM2008 model are used to calculate the gravity anomaly and the vertical gravity gradient, both corrected by the topographic effect, in order to delineate main tectonic features related to density variations. In particular, using the Bouguer anomaly from GOCE, we calculated the crust–mantle discontinuity obtaining elastic thicknesses in the frame of the isostatic lithospheric flexure model applying the convolution method approach. Results show substantial variations in the density, compositional and thermal structure, and isostatic and flexural behavior of the continental lithosphere along the Southern Andes and adjacent foreland region.

The Transitional Zone Between the Southern Central and Northern Patagonian Andes (36–39°S)

The transition zone from the Southern Central to the North Patagonian Andes is characterized by a low topography and low shortenings. During its evolution, an extensional event in late Oligocene times affected the western section of the Late Cretaceous to Eocene fold and thrust belt. Late early Miocene contraction then constructed most of the eastern Andean slope as in sequence structures stacked in the frontal sector of the fold and thrust belt. However, out of sequence structures derived mainly from the inversion of the late Oligocene extensional depocenters uplifted the axial Andean zone at these latitudes. Contractional and extensional stages coincide with periods in which the arc expanded and retracted, respectively. Shortening gradients from 30 km in the north to only 11 km in the south and development of synorogenic depocenters are linked to arc dynamics.

The Late Oligocene–Early Miocene Marine Transgression of Patagonia

The most important Cenozoic marine transgression in Patagonia occurred during the late Oligocene–early Miocene when marine waters of Pacific and Atlantic origin flooded most of southern South America including the present Patagonian Andes between ~41° and 47° S. The age, correlation, and tectonic setting of the different marine formations deposited during this period are debated. However, recent studies based principally on U–Pb geochronology and Sr isotope stratigraphy, indicate that all of these units had accumulated during the late Oligocene–early Miocene. The marine transgression flooded a vast part of southern South America and, according to paleontological data, probably allowed for the first time in the history of this area a transient connection between the Pacific and Atlantic oceans. Marine deposition started in the late Oligocene–earliest Miocene (~26–23 Ma) and was probably caused by a regional event of extension related to major plate reorganization in the Southeast Pacific. Progressive extension and crustal thinning allowed a generalized marine flooding of Patagonia that reached its maximum extension at ~20 Ma. It was followed by a phase of compressive tectonics that started around 19–16 Ma and led to the growth of the Patagonian Andes. The youngest (~19–15 Ma) marine deposits that accumulated in the eastern Andean Cordillera and the extra-Andean regions are coeval with fluvial synorogenic deposits and probably had accumulated under a compressive regime.

Neogene Growth of the Patagonian Andes

After a Late Cretaceous to Paleocene stage of mountain building, the North Patagonian Andes were extensionally reactivated leading to a period of crustal attenuation. The result was the marine Traiguen Basin characterized by submarine volcanism and deep-marine sedimentation over a quasi-oceanic basement floor that spread between 27 and 22 Ma and closed by 20 Ma, age of syndeformational granitoids that cut the basin infill. As a result of basin closure, accretion of the Upper Triassic metamorphic Chonos Archipelago took place against the Chilean margin, overthrusting a stripe of high-density (mafic) rocks on the upper crust, traced by gravity data through the Chonos Archipielago. After this, contractional deformation had a rapid propagation between 19 and 14.8 Ma rebuilding the Patagonian Andes and producing a wide broken foreland zone. This rapid advance of the deformational front, registered in synorogenic sedimentation, was accompanied at the latitudes of the North Patagonian Andes by an expansion of the arc magmatism between 19 and 14 Ma, suggesting a change in the subduction geometry at that time. Then a sudden retraction of the contractional activity took place around 13.5–11.3 Ma, accompanied by a retraction of magmatism and an extensional reactivation of the Andean zone that controlled retroarc volcanism up to 7.3–(4.6?) Ma. This particular evolution is explained by a shallow subduction regime in the northernmost Patagonian Andes, probably facilitated by the presence of the North Patagonian massif lithospheric anchor that would have blocked drag basal forces creating low-pressure conditions for slab shallowing. Contrastingly, to the south, the accretion of the Chonos Archipelago explains rapid propagation of the deformation across the retroarc zone. These processes occurred at the time of rather orthogonal to the margin convergence between Nazca and South American plates after a long period of high oblique convergence. Finally, convergence deceleration in the last 10 My could have led to extensional relaxation of the orogen.

An Introduction to the Southern Andes (33–50°S): Book Structure

This book intends to constitute a useful tool to access to data and a general discussion about the mechanisms that have been associated with the development of the Southern Andes. It is mainly conceived for Earth Science professionals working in academia and industry, as well as Ph.D and Ms students and interested readers in general.

Cenozoic Deformational Processes in the North Patagonian Andes Between 40° and 43°S

The Cenozoic tectonic evolution of the North Patagonian Andes is analyzed linking geological and geophysical data in order to decipher the deformational processes that acted through time and relate them to basin formation. Field observations and seismic reflection profiles reveal the shallow structure of the retroarc area where contractional structures, associated with Oligocene to early Miocene inverted extensional sections, are partially onlapped by early to late Miocene synorogenic deposits. From the construction of five structural cross sections along the retroarc area between 40° and 43° 30′S, constrained by surface, gravity, and seismic data, a shortening gradient is observed along Andean strike. The highest shortening of 18.7 km (15.34 %) is determined near to 41° 30′S where basement blocks were uplifted in the orogenic front area, and the deepest and broadest synorogenic depocenters were formed toward the foreland. Additionally, eastward shifting of Miocene calc-alkaline arc rocks occurred at these latitudes, which is interpreted as indicative of a significant change in the subduction parameters at this time. Deep crustal retroarc structure is evaluated through inversion of gravity models that made also possible to infer Moho attenuated zones. These coincide with the occurrence of younger than 5 Ma within-plate volcanics as well as with crustal thermal anomalies suggested by shallowing of the Curie isotherm calculated from magnetic data.

Active Deformation, Uplift and Subsidence in Southern South America Throughout the Quaternary: A General Review About Their Development and Mechanisms

A broad range of processes act today and have acted simultaneously during the Quaternary, producing relief from the Chilean coast, where the Pacific Ocean floor is sinking underneath the South American margin, to the Brazilian and Argentine Atlantic Ocean platform area. This picture shows to be complex and responds to a variety of processes which are just started to be considered. These processes involve mountains created in a passive margin setting along vast sections of the Brazilian Atlantic Ocean coast and regions located inland, to “current” orogenic processes along the Andean zone. On one hand, mountains in the passive margin seem to be created in the area where the forearc region eastwardly shifts at a similar rate than the westward advancing continent and, therefore, it can be considered as relatively stationary and dynamically sustained by a perpendicular-to-the-margin asthenospheric flow. On the other hand, the orogenic processes associated with the eastern Andes show to be highly active at two particular areas: the Subandean region, where the trench is stationary and the Pampean flat subduction zone to the south, where a shallower geometry of the Nazca plate creates particular conditions for deformation and rapid propagation of the orogenic front generating a high-amplitude orogen. In the Southern Central and Patagonian Andes, mountain (orogenic) building processes are attenuated, and other mechanisms of regional uplift become dominant, such as the (i) crustal weakening and deformation linked to the impact of mantle plumes originated in the 660 km mantle transition, (ii) the retirement of ice masses from the Andes after the Pleistocene producing an isostatic rebound, (iii) the dynamic topography associated with the opening of asthenospheric windows during the subduction of the Chile ridge and slab tearing processes, (iv) the subduction of oceanic plateaux linked to transform zones and (v) the accretion of oceanic materials beneath the forearc region. Additionally and after recent geodetic studies, (vi) forearc coastal uplift due to co-seismic and post-seismic lithospheric stretching associated with large earthquakes along the subduction zone, also shows to be a factor associated with regional uplift that needs to be further considered as an additional mechanism from the Chilean coast to presumably the arc zone.

Multiple geophysical methods used to examine neotectonic structures in the western foothills of the Sierra de El Maitén (Argentina), North Patagonian Andes

A high-resolution geophysical study was carried out in a region of the retroarc of the Patagonian Andes located on the western slope of the Sierra de El Maiten. This structure is characterized by an imbricated west-vergent fault system developed in the orogenic front area of the North Patagonian Andes that has uplifted. Oligocene volcanic rocks (Ventana Formation) affect Miocene to Quaternary sediments. Even though neotectonic fault scarps are affecting Quaternary deposits in the foothills of this range, no direct observation of slip in Quaternary strata was determined. The main objective of this study is to determine geometry of recognized neotectonic structures, characterizing them by variations in magnetic susceptibility, density, and p-wave velocities. The combined application of different geophysical methods has allowed the characterization of the bedrock geometry and the determination of neotectonic displacements along faults. The potential field model and its integration with a seismic profile show the accurate geometry of this tectonic zone, which is crucial for seismogenic hazard analysis, in the area of northern Patagonia, a highly significant economic zone due to tourism with several towns (El Maiten, Esquel, and San Carlos de Bariloche) dispersed throughout the area of young tectonic activity.